I. Field of the Invention
The present invention relates generally to RF receivers using quadrature demodulation. More particularly, the present invention relates to a novel method and apparatus for providing automatic gain control, out-of-band signal rejection, and D.C. offset cancellation within a digital receiver.
II. Description of the Related Art
In analog receivers, such as are used in narrowband FM cellular communication systems, FM demodulators are employed to extract information encoded in the phase of an incident waveform. Existing FM demodulators often include an analog frequency discriminator preceded by an analog limiter, with the limiter serving to constrain the input signal power to a constant level. In this way maximum signal to noise ratio is maintained at the input to the frequency discriminator over the full dynamic range of the FM input signal. However, such an analog signal processing technique generally involves extensive signal filtering, and frequently is implemented using a large number of discrete components. Moreover, it has been demonstrated that improved performance may be achieved using linear digital waveform demodulation rather than analog demodulation. Unfortunately, conventional demodulation techniques are often not applicable to digital receivers, since clipping of the received signal would result in corruption of the data derived therefrom.
A digital receiver for receiving a digitally modulated information signal will generally include a variable gain amplifier with a gain adjusted by a control signal. The process of adjusting the gain of a received signal using a control signal is called Automatic Gain Control (AGC). Typically in digital receivers, the AGC process involves measurement of an output signal power of the variable gain amplifier. The measured value is compared with a value representing the desired signal power and an control signal for the variable gain amplifier is generated. The error value is then used to control amplifier gain so as to adjust the signal strength to coincide with the desired signal power. To effect digital demodulation with an optimal signal to noise ratio, automatic gain control is used to hold the magnitude of the baseband waveforms dose to the full dynamic range of the baseband analog to digital converters. This generally requires, however, that automatic gain control be provided over the full dynamic range of the received signal power.
In the cellular environment, a digital receiver may receive a signal which experiences rapid and wide variations in signal power. In digital receivers such as are used in a code division multiple access (CDMA) and Time Division Multiple Access (TDMA) mobile cellular telephone, it is necessary to control the power of the demodulated signal for proper signal processing. However, in digital receivers to be both CDMA or TDMA compatible and conventional FM compatible, i.e., dual-mode digital/FM receivers, it is necessary to provide power control of both wideband CDMA (or TDMA) signals and narrowband FM signals. The control process is complicated by the differing dynamic ranges associated with the received FM and CDMA signal power. That is, the magnitude of received FM signals may vary over a dynamic range greater than 100 dB, whereas CDMA systems typically result in a more limited dynamic range, i.e., approximately 80 dB.
The provision of separate AGC circuitry for each mode increases the hardware complexity and expense of such receivers. Accordingly, it would be desirable to provide AGC circuitry capable of operating both upon narrowband, wide-dynamic range FM signals, as well as upon wideband CDMA signals of more limited dynamic range.
It would also be desirable to provide digital AGC in inexpensive receivers utilizing analog to digital (A/D) converters with limited dynamic range. Again, because FM signals within cellular systems may vary more than 100 dB and relatively inexpensive 8-bit A/D's are limited to a dynamic range of approximately 48 dB, a cost effective AGC implementation should be capable of controlling the gain of the portion of the receiver preceding the A/D converters so as to control the signal's dynamic range at the A/D converter. The alternative is to employ expensive A/D converters having greater dynamic range, thereby increasing the cost of the receiver or to increase the AGC range of the analog portion of the radio which is very difficult and costly.
It is therefore an object of the present invention to provide a novel and improved AGC circuit which incorporates the desirable features mentioned above, and which, as is described hereinafter, also realizes certain other advantages relative to conventional AGC techniques.
In standard FM cellular telephones, the AGC function is performed by a circuit called a limiter. When a limiter is used, out-of-band signal rejection can only be done using intermediate frequency (IF) filters. Although the requisite signal rejection capability may be achieved through the use of ceramic IF filters, these tend to be relatively large and expensive. Smaller and less expensive IF filters are generally incapable of being realized so as to possess the desired signal rejection characteristics, and hence are generally not employed in FM cellular telephone receivers.
As is well known, recent advances in integrated circuit (IC) technology have made possible the realization of active baseband filters which are quite small and inexpensive compared to IF filters. It follows that it would be desirable to employ active IC baseband filters to effect significant out-of-band signal suppression, thereby allowing smaller and less expensive IF filters to be used to provide any additional required signal rejection. In an active filter, the higher the gain--the better rejection that is possible. But the higher the gain, the more susceptible the system to unwanted D.C. offsets. Suppression of such D.C. offsets is desirable to maximize the available signal dynamic range, minimize offset induced distortion in the baseband demodulated signal and minimize offset induced errors in baseband signal strength estimates.
In standard digital communications systems such as quadrature phase shift keying (QPSK), used in standard CDMA communication systems (and some TDMA systems), or binary phase shift keying (BPSK), information from the waveform is recovered by downconversion of the signal to baseband frequency centered about D.C. In this case D.C. offsets are easily removed, since for QPSK and BPSK, the carrier is generally suppressed by the transmitter anyway. Hence at baseband, a D.C. notch can be used.
However, for constant amplitude modulations such as FM and continuous phase FSK (which are used in FM cellular telephone systems such as AMPS) and Gaussian Minimum Shift Keying (GMSK) (used in some TDMA systems), the carrier must be preserved in order to demodulate the received signal.
The employment of active baseband IC filters leads to the necessity of providing some mechanism for suppression of undesired D.C. offsets. The IF processing chain of conventional digital cellular telephone receivers typically includes a local oscillator (L.O.) having a frequency selected such that the carrier frequency is downconverted to D.C., and a simple D.C. notch filter is used to remove unwanted D.C. offsets. If an FM, FSK, or GMSK signal is processed by such an IF processing chain, then the D.C. offset suppression will not only remove unwanted D.C. components, but also critical phase and amplitude information at the carrier frequency. That is, in FM cellular telephone systems significant amplitude and phase information is present at the carrier frequency, and performance will be adversely affected if such information is destroyed.
However, there are two narrow bands of frequencies in between the carrier frequency F.sub.c and F.sub.c +F.sub.1 and between F.sub.c and F.sub.c -F.sub.l (where F.sub.l is the lowest frequency expected in the demodulated spectrum, typically F.sub.1 =300 Hz for FM cellular) which can be suppressed without adversely affecting the demodulated signal. Although minimal voice information is carried at intermodulation products at frequencies close to the carrier frequency, such products are uncommon and of relatively short duration. Accordingly, the suppression of only the low-frequency intermodulation products after baseband downconversion does not usually result in the loss of appreciable voice information. Similarly, in FSK and GMSK systems, very little signal power is present below F.sub.l =(symbol rate)/100, so again the frequency band between F.sub.c and F.sub.c +F.sub.l may be suppressed without degradation of the digital data.
It is therefore a further object of the present invention to provide quadrature receiver in which high-gain/highly selective active baseband filters may be employed without causing the loss of carrier frequency information.